Shock absorbers which force fluid through a restricted orifice to convert the kinetic energy of a moving part into an increase in the thermal energy of the fluid are commonly used on various types of machines and are therefore well known in the art. The smoothest deceleration of the moving part is obtained by absorbers which offer a constant resistive force to the motion over the total length of the deceleration.
One type of such devices employs a piston connected to the machine part and movable within a cylinder having a closed end. A series of exponentially spaced holes are formed along the length of the cylinder wall, and the cylinder is supported within a housing filled with fluid. As the piston is forced into the cylinder by the motion of the machine part, the fluid forced through the holes and the kinetic energy of the part is converted into thermal energy of the fluid. As the piston moves down the cylinder, it successively closes off the holes so that the force imposed on the load is maintained relatively constant, resulting in a linear deceleration of the moving part.
The force imposed on the part is a function of the configuration of the fluid orifice and linear decelerators of this type have been designed wherein the orifice configuration may be varied to accommodate the device for use with parts having varying weights and kinetic energy. One common approach is to provide grooves in a tubular sleeve fitting over the cylinder. The grooves in the sleeve cooperate with holes in the cylinder to define the fluid orifices. The angular or axial position of the sleeve on the cylinder may be adjusted to vary the orifice configuration and thus the resistance provided to the load. Representative examples of so-called "groove-on-hole" shock absorbers are disclosed in commonly assigned U.S. Pat. Nos. 4,059,175, 4,298,101, and 4,321,987, as well as U.S. Pat. No. 3,425,522 to Gryglas and U.S. Pat. No. 3,693,767 to Johnson. More recently, an improved groove-on-hole design has been devised as disclosed in U.S. Pat. No. 4,702,355 assigned to the assignee of the present invention, which advantageously employs a semicircular groove to create substantially turbulent flow at the metering orifice. Finally, also of some interest is U.S. Pat. No. 4,482,035, likewise assigned to the assignee of the present invention, which discloses a hole-on-hole type of cylinder and sleeve arrangement.
In connection with the prior art shock absorber designs described above, the ends of the outer tube of the shock absorber were closed by means of end walls or caps secured in place by snap rings. It was discovered that such snap rings allowed slight circumferential movement of the inner tube or cylinder relative to the metering sleeve, thus creating improper alignment of the holes in the cylinder relative to the holes in the metering sleeve. In order to solve this problem, grooves were formed in the inside surface of the metering tube, each of such grooves being provided with an exhaust hole somewhere along its length, thus creating the "groove-on-hole" arrangement described above. The groove-on-hole arrangement is less than completely desirable, in part because of the sheer expense of forming a plurality of properly configured and dimensioned grooves in the inside surface of the metering sleeve. Also, such an arrangement, because of its geometric configuration, resulted in some degree of erosion of those portions of the sleeve/cylinder against which the hydraulic fluid flows, as it Passes from an area of high pressure, through the metering holes, into an area of low pressure. The erosion problem was, of course, much more severe in those applications in which the shock-absorbing device was required to absorb the force of relatively large loads over a large number of repetitions.
The use of snap rings to mount the end walls of the outer cylindrical tube, in addition to permitting undesired rotation of the inner tube, sharply limited the rating or capacity of the shock-absorbing device, since they were effectively the "weak link" in the shock-absorbing mechanism when operating under heavy loads. The high forces generated by the machine element bearing on the piston and cylinder result in fluid pressures that are sufficiently high to overcome the restraining force of the snap rings, and thus blow out one of the end walls.
The present invention is directed to overcoming each of the deficiencies mentioned above.